Modification of Polyp :-
(1) Gastro zooids/ Tropho zooids/ Nutritive zooids :-
(i) Cylindrical with large mouth and a tentacles.
(ii) Tentecles bear lateral branches the tenilla with nematocyst.
(iii) A well formed mouth & gastro vascular cavity.
(iv) Function - food ingestion.
(v) Variation - No tentacles in gastro zooids of velella.
Gastro zooids exists in the following modified forms.
(A) Siphon :-
(i) It is a polyp form, but without normal tentacles.
(ii) A single large hollow, highly contractile tentacle hang from or near the base.
(iii) Lateral contractile branches of tentacles are called tenilla, which terminates in a large & complicated know of nematocysts.
(B) Siphono zooids :-
(i) Gastro zooids is modified to produce a current producing device at the expense of other structure.
(ii) They are devoid of tentacles, longitudinal muscles but may sometimes possess septal filaments.
(iii) The siphonoglyphs are strongly developed.
Examples :- Pentularia; Renilla;
(2) Dactylo zooids/Macro zooids :-
(i) Protective zooids are actually derived from the gastro zooids by the reduction or total loss of mouth.
(ii) They are elongated & highly extensible.
(iii) Long basal unbranched tentacle arising from the base of the polyp and without nematocysts.
(iv) Funtion - Tactila & projection of colony.
The Dactylozooids exhibit following structure variations-
(A) Tentaculo zooids :-
(i) Dactylozooids assumes a long tentacle like appearance.
(ii) They may be definity arranged in relation to the gastrozooids & are usually situated at the margin.
(iii) Function - Sensory in function.
Example :- Hydroctena;
(B) Spiral zooids :-
In Hydroctena the spiral zooids with capitate tentacles remain scattered through out the colony.
(C) Sarcostyles / Nematophore :-
(i) Sarcostyles spring from tiny theca located on stems and on the hydrothecae of gastro zooids.
(ii) The nematophores are usually with capitate ends, beset with nematocysts or adhesive cells or both.
Example :- Plumularidae;
(D) Palpons :-
(i) Simple hollow tentacle like bodies which spring from the margin of the body.
(ii) Palpons in velella & Porpita act as the Dactylo zooids.
(E) Cyston :-
(i) In Siphonophora, a distal pore present in the dactylo zooids is called the cyston.
(ii) Function - Excretory in function.
(3) Gonozooids / Blastostyles :-
(i) Club shaped bodies devoid of mouth & tentacles.
(ii) The coelenteron is greatly reduced.
(iii) Modified polyps that produce medusoid forms or their morphological equivalent by budding.
(iv) Function - These zooids are the reproductive individual of the colony.
The Gonozooids exhibit following structure variations-
(A) Gonosiphon :-
(i) Gonozooids may resemble gastrozooids & may even possess a mouth.
(ii) The tentacles are absent.
Examples :- Velella; Porpita;
(B) Gonodendron :-
(i) Gonodendrons are present as brancheol stalks which bear grape like clusteres of gonophores & are usually provided with a long retractile gonopalpon.
(ii) Found in Siphonophora.
(C) Gonopalpon :-
(i) Tentacle like dactylo zooids remains associated with gonophores.
(ii) Found in Siphonophora.
Special Types of Zooids :-
(A) Gonostyles :-
(i) The mouth & coelenteron are present.
(ii) Gonads remain attached with the Siphono zooids.
(iii) These are secondary siphono zooids. example- Porpita;
(iv) Function - Gonostyles are primarily reproductive in function & may secondarily be nutritive.
(B) Hydrorhiza :-
In Obelia hydrorhiza acts as the organ of attachment for the whole colony.
(C) Hydrocaulus :-
In Obelia, the hydrocaulus, arising from the hydrorhiza, bears different zooids & helps to convey the food matters to the different parts of the colony.
Modifications of Medusoid Form :-
(1) Nectocalyx / Nectophore / Swimming bell :-
(i) This form is present in siphonophora excepting physalia. Typically medusoid with bell, vellum, radial canal etc. But lack mouth-manubrium sense organ & tentacles.
(ii) Muscles highly developed.
(iii) Function - Locomotion of colony.
(2) Bract / Hydrophyllium / Phylozooids :-
(i) Highly modified, thick geletinous leaf like or helmet shaped with simple or branched radial canal.
(ii) Function - Protection of the colony.
(3) Pneumatophore / Float :-
(i) This type is present in siphonophora excepting Diphyes.
(ii) Medusoid bell, Muscular exumbrellar wall called Pneumatocodon & glandular subumbrellar wall known as Pneumatosaccus or Air sac with opening.
(iii) This opening may be closed or reduced to a pore guarded by a sphincter muscle.
(iv) Gastrovascular cavity divided into chambers. Bottom of the air sac form expanded funnel producing gas called gas gland. In Physalia gas contain 91% Nitrogen, 1.5% Argon & 7.5 % Oxygen.
(v) Function - Hydrostatic.
(4) Auriophore :-
Function unknown. Ovoid in shape with a minute canal connect to pneumatophore with which it always remain associated. Around canal is a mass of muscles called Pistillum.
(5) Gonophores :-
These are the medusoid forms which are sexually differentiated.
(i) They may occur singlely or in clustures.
(ii) Most of the colony are hermaphrodite. Gonophores are diocious. They may be medusa like structure having bell, vellum, radial canals & a manubrium, Radial canal bears gonads.
(iii) The gonophores are devoid of mouth, tentacles & sense organs. In some cases the female gonophores appears as medusa like, while the male gonophore sac like. The male gonophore may attached with the colony while the female gonophore die after discharging the sex cells.
(iv) Function - Reproduction.
